People with slender lower faces usually have lower jawbones that bite slightly backward, giving them a "convex facial profile" and overbite.

Slender jaws are also associated with higher risks of tuberculosis, an infectious disease that affects the lungs and can be symptomless. A third of the world's population is believed to have a latent form of TB, with 9.6 million cases reported in 2014. More than 95 percent of TB deaths occur in developing countries.

"Almost 2,000 years ago a Greek physician was first to identify slender jaws as a marker for TB susceptibility, and he turned out to be right!" said University of Washington professor of dentistry and adjunct professor of epidemiology Philippe Hujoel. "Twentieth-century studies confirmed his clinical observations, as slender facial features became recognized as one aspect of a slender physique of a TB-susceptible person."

The theory may explain certain geographical coincidences. The United Kingdom reports elevated levels of TB, as well as higher numbers of left-handed people and people with slender faces.

However, 19th century Eskimo populations that had "robust" faces and were depicted in artwork as right-handed were generally "tuberculosis-resistant."

"In a world dominated by an obesity crisis and right-handers, ectomorphs can be different in their desires," Hujoel said. "Popular websites suggest they commonly express a desire to gain weight or muscle mass. Their slightly increased chance of being a 'leftie' is an additional feature that makes them different."